Field of Endeavor
The present application relates to desalination and more particularly to phased charging and discharging in capacitive desalination.
State of Technology
This section provides background information related to the present disclosure which is not necessarily prior art.
Water purification—the rendering of non-potable water into water fit for human consumption and use—may involve many processes including desalination, disinfection and decontamination. Desalination technologies typically fall into three categories: reverse osmosis (RO) membrane filtration, multi-stage flash distillation, and electrokinetic desalination methods, such as capacitive deionization (CDI). Unlike CDI, RO and distillation are subject to high equipment capital, operation, and maintenance costs, which are associated with required high fluid pressure or temperature to desalt.
CDI is a relatively new technology known as a robust, energy efficient, and cost effective technology for desalination of water with a low or moderate salt content. It requires low voltage (˜1V) power supply for removing salt and low pressure pump for delivering feedwater to the system. The active component is a pair of specially engineered porous carbon electrodes. Upon applying potential (charging step), salt ions are removed from the feed water and held electrostatically in electric double layers (EDLs) along the pore surfaces. Once the EDLs are fully charged, the clean water is pushed out of the system. The system is then regenerated by removing the voltage (discharging step), allowing salt ions to spontaneously release from the pore surfaces and into the feed, forming a brine.
CDI systems are mostly operated in single- or multiple-pass continuous flow mode in which the throughput and permeate recovery are coupled. This operational mode is useful in the case of constant feedwater properties, e.g. ionic composition, as the quality of effluent stream is more predictable. Stopped flow or batch mode desalination is another operating scenario in which the salt solution is quiescent and charging and flow stages are sequential rather than simultaneous. In both cases, the effluent concentration varies with time during adsorption and desorption steps.
During constant voltage charging step in continuous flow mode, effluent concentration drops quickly at first and reaches a minimum. Then it slowly increases since (1) the electrode adsorption rate decreases and (2) the new solution displaces the desalted solution. Similarly during discharging step, concentration increases sharply due to high desorption rate at the beginning and then starts to slowly decrease as the brine is replaced by feedwater. This slowly varying concentration profile is not desired; as it extends the charging/discharging cycle time and lowers the overall permeate recovery. In other words, the conventional method uses more energy per produced freshwater and it produces more brine (waste).